Process of making diphenyl



Patented Dec. 20, 1952 UNITED. "STATES,

PATENT OFFICE I THOMAS E. WARREN, OI OTTAWA, ONTARIO, GA'NADA, AND CHARLES B. mmem, OI

ANNISTON, ALABAMA, ASSIGNOBS 'IO BWANIN' RESEARCH, INC A CORPORATION OI ALABAMA I OF MAKING DIPHENYL Application med January 0, mo. serial No. 419,059.

This invention relates to an improvement in a method and apparatus for making (11- phen 1 from benzol y thermal s nthesis, and has or its object to provide or mcreased 5 yields 'of diphenyl and economies 1n the heat r uirements forsuch a process.

ll further object of our invention is to provide a process and apparatus of the character designated wherein the occurrence of s1de reactions and the production of tar and carbon is minimized. It has recently'been discovered that d1- phenyl in commercial quantities may be made by passin benzol through a bath of molten metal suc as lead. In the operation of this process on a lar e scale we have encountered difiiculties w ich are inherently connected with such large-scale operation,

and which only became apparent when such large-scale factory operatlons were undertaken.

The difliculties 'which were thus encountered were the following :We have found that when heating large containers of molten metal to temperatures in the neighborhood of the diphenyl formation temperature, namely, between 600 C. and 850 (1., the hydrostatic head of metal caused strains to be set up in the metal containers which finally caused leaks and loss of m tal, with considerable monetary loss. Furthermore, with metals there is always the danger of alloys being formed which will further weaken the metal container.

These difiiculties have been overcome by our discovery that certain molten salts could be used in place of the molten lead or other metal without subjecting the container to {dangerous strains at high temperatures and without alloys being formed between the metal of the bath and the metal container.

We have further found that, in general, molten salts do not accelerate carbon formation as do metals, and hence have been able.

to so modify our procedure as to prevent a considerable loss through decomposition of the benzol used. 7

The salts which we have found workable are calcium chloride, mixed with suflicient sodium chloride so that its melting point is lowered to the neighborhood of 600 C. We have found that a mixture of 28% sodium chloride and 72% calcium chloride is particularly well suited for thiswork, since it has a low melting point. Other salts which we have used are sodium meta-phosphate, or sodium pyrophosphate, which on heating to the dlphenyl temperaturechange to sodium meta-phosphate. We have also used a mixture of of sodium carbonate-and potassium chloride, with satisfactory results. All of these salts have densities substantiall less than 2.5 at ordinary temperatures, whic isjurther decreased on heating and melting.

As a result of the use-of molten salts in place of molten metals, we have obtained igher yields of diphenyl in our process. We believe that these higher yields are due to the fact that the differences in density be tween the molten salt and the benzol vapor is less than the difference in density between the molten metal such as lead, and the benzol vapor. This lesser difference in density permits the vapor toremain longer in contact with the salt and hencepermits a nearer approach to the chemical equilibrium to be attained.

While we do not at present understand the exact mechanism of the'diphenyl reaction, we have found that certain metals cause side reactions to take place in the benzol vapor phase, the final result of which is the production of carbon and hydro en. Our experience with fused salt baths mdicates that these objectionable side reactions do not occur to so great an extent, and that consett uently the benzol vapor may be maintained or a longer period of time at diphenyl forming temperatures without large quantities of carbon and hydrogen being formed. This 90 longer period of time permits the diphenyl forming reaction to approach substantial equilibrium. 7

Our method is preferably carried out as illustrated in the accompanying drawing, in 95 which Fig. 1 is a flow sheet showing one means of carrying the invention into effect, and

Fig. 2 is a similar view showing a modified manner of carrying out the invention.

coil, during which passage it absorbs heatfrom the walls of the pipe. The lower end of the coil is perforated and the vapor streams out and in rising through the molten salt, in physical contact therewith, agitates the bath in such a manner as to maintain a pract cally uniform temperature throughout the bath and reaches the temperature of the bath by.

the time it passes therefrom. t

By employing a large mass of molten salt we are enabled to prevent overheating of the vapor due to the heat capacity of the bath and by means of a thermocouple immerse therein to observe temperature changes which may occur and change the heating so as to' supply additional heat or lessen the rateof heating as may be desired. We prefer to operate the preheater bath at a temperature of 600 to 650 C., and have found from experience that this is the optimum tempera ture range for continuous operation- After the preheated vapor emerges from the preheaterbath, it is immediately passed into a converter bath which is of similar design and arrangement as the preheater. The

vapor passes to the bottom of the bath and streams out through holes in the distributor, and then bubbles up through the salt in a manner similar to the preheater. The converter bath is kept at a temperature range of from 750 to 850 C. When the vapor passes from the converter bath, it is passed to a condenser Where the benzol and diphenyl are condensed while-the hydrogen which is liberated is suitably vented to the atmosphere.

In view of the fact that we have found that metals, particularly iron, cause considerable carbon formation from benzol at diphenyl forming temperatures, it is desirable if iron apparatus is employed to so construct the passages and the condenser that rapid cooling of the vapor is efi'ected. Furthermore, because of the fact that the use of molten salts particularly decreases the rate of carbon formation from benzol, it is possible to operate without a preheater, it being however desirable to vaporize the benzol first, although this is not absolutely necessary when operating at low rates.

In some instances it may be desirable to cool the benzol diphenyl vapors leaving the. diphenylconverter, by passing them through a molten salt bath maintained at temperatures below the diphenyl forming temperatures. This method of operating is illustrated in Fig. 2 of the drawing and is particularly desirable where the preheater an cooler may be combined in one unit. In this case the benzol vapors being preheated are passed through a continuous pi 10 immersed in the preheating salt bath, t en bubbled through a converter salt bath maintained at diphenyl forming temperatures, after which the vapors return to the preheating salt bath through a pipe 11, being now however bubbled through the molten salt itself. By working in this manner,- the hot diphenyl containing vapors are caused to impart their excess heat to the molten salt of the effective manner, while the incoming benzol vapors acquire the heat given up by the outgoing gases. This vides for the carrying out of the dipheny reaction and the cooling of the vapor out of contact with metals, consequently avoids material loss through the formation of carbon, as well as'eifecting a considerable economy in heat recovered. k

While we have shown our invention in but two forms, it will be obvious to those skilled .in the art that it is not so limited but is sus-' ceptible to various other changes and modifications, without departing from the spirit thereof, and we desire therefore that only such limitations shall be placed thereupon as are imposed by the prior art, or as are specifica'lly set forth in they appended claims.

What we claim is:

1. In a process of producing diphenyl, passing benzol vapors through a molten inorganic salt having a melting point less than the diphenyl forming temperature, and inert with respect to the benzol vapors and heated to a temperature at which substantial quan tities of diphenyl are formed, and condensing the vapors.

2. In a process of producing diphenyl,

passing benzol va ors through an inert molten salt bath, said salt having a density substantially less than 2.5, a melting point less than the diphenyl forming temperature, and heated to a temperature at which substantial quantities of diphenyl are formed, and cooling and condensing the vapor. 3. In a process of producing diphenyl, passing benzol vapor through a molten salt, said salt having a melting point less than the diphenyl forming temperature and inert with respect to the bQllZcl vapor.

4. In a process of producing diphenyl, passing benzol vapor through a molten salt heated to diphenyl forming temperatures, and then through a molten salt heated to temperatures substantially less than diphenylforming temperatures.

5. In a process of producing diphenyl, passing benzol vapor through a preheater molten salt bath maintained at lessthan diphenyl forming temperatures and out of direct contact therewith, then through a molten method of working ro-' preheater bath in a very salt bath maintained at diphenyl forming temperatures and then throu h said preheater bath in direct contact t erewith.

In testimony whereof we, Tnoms E. Wu- 5 mm and CHARLES B. Dtmom, aflix our signatures.

THOMAS E. WARREN. CHARLES B. DURGIN. 

